Conventional sash locks for double hung windows typically are formed in two principal parts, a keeper part that is fixed to the upper sash and a locking part that is fixed to the lower sash. The locking part includes a housing mounting a rotatable locking cam and an operating lever to rotate the locking cam between “locked” and “unlocked” positions. In the “locked” position, the locking cam engages the keeper part and prevents opening movement of the two sashes. In the “unlocked” position, the locking cam is retracted into the housing, allowing the sashes to be opened and closed. The Mosch U.S. Pat. No. 4,736,972 is representative of such devices.
For a time, sash locks were made entirely of metal. More recently, however, there has been an effort to convert to plastic materials, which are lighter in weight and less expensive. The Piltingsrud et al U.S. Pat. No. 5,161,839, for example, utilized a glass-filled nylon material to form a positioning spring to retain the locking cam in its “locked” and “unlocked” positions. In the Murphy et al U.S. Pat. No. 6,568,723 most of the parts of the sash lock were constructed using glass-filled polypropylene and/or glass-filled nylon, while forming the locking cam itself of metal, preferably zinc. The Chaput U.S. Pat. No. 5,741,032 and the Miller et al U.S. Pat. No. 7,665,775 are examples of sash lock devices formed entirely of plastic material.
Heretofore, at least certain of the sash lock manufacturers have felt the need to utilize metallic detent springs, rather than plastic springs, in otherwise all-plastic sash lock devices, in order to provide a strong and long-lasting detent mechanism for retaining the sash locks in their operative “lock” and “unlock” positions and also to provide a strong snap-action effect during movement of the locking cam from one position to the other. However, the use of metallic detent springs complicates the manufacturing and assembly operations and otherwise adds unwanted cost to the product.